The present invention relates to improved compositions and methods used for the extraction and recovery of metals from starting materials such as mineral ores, recyclable wastes, contaminated soils, toxic wastes such as dusts producing through steelmaking processes (such as electric arc furnace dusts and BOP dusts), and other materials. More particularly, the present invention relates to a methodology using caustic silica solution compositions that enhance the extraction and recovery of metals from mineral ores, recyclable wastes, contaminated soils, toxic wastes such as dusts producing through steelmaking processes (such as electric arc furnace dusts and BOP dusts), and other materials.
A wide variety of methods for chemical extraction and/or recovery are known in the art, particularly in regard to extraction and recovery of metals from materials such as mineral ores, recyclable wastes, and contaminated soils. Such methods include, for example, amalgamation that often produces health hazards and clean-up hazards; flotation that requires finely ground, de-slimed, clean, free metal; cyanide treatment that offers specific extraction, but presents environmental hazards; centrifugal concentration which works primarily for free on heavy metals; electrolytic/electrowinning processes that are expensive and slow; thermal/furnace processes that are energy intensive and expensive; and chlorination/bromination processes that require high pressure digestors that are, in turn, expensive, hazardous, and leak-prone.
Representative of the prior art, Rizet, in U.S. Pat. No. 5,549,811, teaches a process for decontamination of soils polluted with metals, wherein the polluted soil is treated with a NaOH solution to remove contaminant metals. By leaching this contaminated soil with a soda-solution containing a concentration of about 6N NaOH, the contaminants are precipitated in the form of hydroxides metals and then extracted via simple solid-liquid separation techniques. Thus, Rizet attempts to prepare contaminant metals for subsequent extraction stages well known in the art.
It will be apparent to those skilled in the art that the extraction solution taught by Rizet contains 2% hydrogen peroxide, an oxidizer, and 25-30% sodium hydroxide. His procedure includes the application of lime in a sufficient quantity to remove silica from the extract solution by effectuating precipitation of aluminosilicates that may have become solubilized. Also included is a second wash of the soil that contains the precipitated aluminosilicates with a 6N soda-solution and pH of 14, thereby tending to redissolve some of the silica that has been precipitated by the lime. By sustaining a basic medium with a pH of 14, Rizet avoids the precipitation of lead in the form of hydroxide. Besides the recovered lead, silica remains in the wash liquid.
Also indicative of the art is European Patent No. 34137 in which Reiterer discloses a process for the hydrometallurgical treatment of materials that contain zinc, wherein pulverized zinc-containing materials are initially subjected to basic leaching with alkali hydroxide. Precipitate is separate from the liquid, and the liquid is diluted with water and acidified to a pH below 7 in order to precipitate SiO. Prior to this SiO precipitation, Clxe2x88x92 and NH+ ions are introduced into the solution. Then, once Cu and Cd have been separated, the Zn-containing solution is subjected to an ion exchange or solvent extraction process, and, if necessary, to an electrolytic purification process.
Since Rizet discloses that approximately 1-3 g/l silica is present in the post-leaching solution, the Examiner concludes that Applicant""s caustic silica solutionxe2x80x94inherent in Applicant""s chemical processing stepxe2x80x94and concomitant treatment methodology would have been obvious to those of ordinary skill in the art.
Each of these approaches use an initial alkali-based leaching step to attempt to solubilize the metal contaminant portion and the like so that it may be extracted from the contaminated material. Unfortunately, as will be appreciated by those skilled in the art, only limited extraction of metals from ores, dust, and the like has been hereinbefore obtained using conventional separation methods. It would be advantageous for promoting extraction of metals from such contaminated materials if a leaching solution and concomitant methodology were developed that is capable of achieving levels of metal extraction heretofore unknown in the art.
The present invention overcomes many of the disadvantages of known extraction and recovery methods by providing liquor compositions that, while leaching the contaminated materials under mild conditions, effect a chemical change in the underlying structure of the contaminated materials wherein a permanent proclivity for metal extraction and recovery is attained. The present invention performs this extraction and recovery function while generating relatively innocuous by-products or wastes.
One aspect of the present invention pertains to the use of liquor compositions for the extraction of preferably a metallic element from a metal-contaminated starting material comprising solidsxe2x80x94containing this elementxe2x80x94by effectuating preferably prolonged contacting of such starting material with the liquid or liquor to cause the underlying structure of the starting material to be broken down to an extent heretofore unknown in the art. As will be hereinafter described, this breakdown of the underlying, typically complex structure, renders the contaminated starting materials to be more susceptible to metal separation than has been hereinbefore believed possible. Once the starting material has thus been properly chemically treated preferably with a caustic silica solution as will be hereinafter described in detail, a mixture of solids and liquid is formed, thereby effectively solubilizing the metallic element in the liquid, wherein a liquid extract may be readily separated from this mixture of solids and liquid.
It will become clear to those skilled in the art that the compositions of caustic silicate solution of the present invention correspond essentially to saturating levels of silica. It has been discovered that the presence of such a constituted caustic silicate solution afford a synergy of chemical and physical properties that apparently vigorously attack the formerly stubborn, hard-to-crack underlying structure of the silica-containing starting material.
In some preferred embodiments, this caustic silica solution comprises 0.001% to about 5% w/w dissolved silica. In some preferred embodiments, this caustic silica solution comprises silica; and one or more of sodium hydroxide, potassium-hydroxide, and ammonium hydroxide. In some preferred embodiments, the caustic silica solution comprises silica; and one or more of sodium hydroxide, potassium hydroxide, and ammonium hydroxide. In some preferred embodiments, the caustic silica solution comprises silica; and an alkali metal hydroxide. In some preferred embodiments, this caustic silica solution comprises silica; and one or more of sodium hydroxide and potassium hydroxide. In some preferred embodiments, this caustic silica solution comprises silica; and sodium hydroxide. In some preferred embodiments, the caustic silica solution comprises silica and 1-60% w/w sodium hydroxide.
In some preferred embodiments, the leaching or contacting is performed at a temperature of 10-200xc2x0 C. for a period of 10 minutes to 6 hours. In some preferred embodiments, this leaching or contacting is performed at a temperature of 10-200xc2x0 C. for a period of 10 minutes to 6 hours under a pressure of 0.1 to 5 MPa. In some preferred embodiments, this starting material comprises a mineral ore, soil, toxic waste, or dust produced through steelmaking processes. As will be appreciated by those skilled in the art, some preferred embodiments are designed to perform the extraction of the present invention upon starting material that comprises dust produced by an electric arc furnace. Some preferred embodiments are designed to perform the extraction of the present invention upon starting material that includes metal as a contaminant. Some other embodiments are formulated to perform the extraction of the present invention upon starting material that includes an element selected from the group consisting of heavy metals, noble metals, platinum group metals, and toxic metals. Similarly, other embodiments are formulated to perform the extraction of the present invention upon starting material that includes an element selected from the group consisting of Pb, Au, Cd, Zn, As, Ba, Cr, Hg, Se, Ag, Pt, Ti, V, Mo, Zr, and Pd. It will be clear to those skilled in the art that some preferred embodiments are designed to sustain the chemical breakdown of the starting material such that recovery of the solubilized metallic element from the liquid extract is maximized. In a manner well known in the art, once the chemical and physical breakdown taught by the present invention is achieved, then maximum metal recovery may be, in turn, achieved by precipitation of insoluble salts, electrowinning, or electrodeposition.
Another aspect of the present invention pertains to caustic silicate compositions that enable efficient and substantially complete extraction of a metallic element from a starting material comprising solids. As will be understood by those skilled in the art, once the underlying structure is broken down during the typically protracted leaching with a liquid caustic silica solution taught by the present invention a mixture of solids and liquid is formed, thereby chemically altering these solids whereby such solids may be separated as a solid residue from this mixture of solids and liquid. It should be evident that this solid residue contains the metallic element, and that recovery of this metallic element from the solid residue is then routinely attained.
These and other objects and features of the present invention will become apparent from the following detailed description.
The present invention provides compositions of caustic silicate solutions intended for a contacting and leaching step of a diversity of methodologies applicable for the extraction and recovery of a plurality of elements, more preferably for the extraction and recovery of a plurality of metals, still more preferably for the extraction rho and recovery of a plurality of metals such as lead, gold, cadmium, and/or zinc, from starting materials containing a plurality thereof. As is well known in the art, such methodologies typically commence with the step of leaching contaminated material to render certain impurities and contaminants susceptible to commonly used subsequent extraction and removal techniques. U.S. Pat. No. 5,549,811 exemplifies this common approach to separation of metallic elements and the like.
The present invention provides a new genre of contacting solutions that besides the conventional removal of soluble constituent elements from contaminated starting material or the like, chemically alters the underlying structure of this starting material, thereby significantly enhancing the extent to which certain elements are solubilized. It has been found that, by aggressively breaking down the typically complex and xe2x80x9chard-to-crackxe2x80x9d underlying structures of starting materials such as mineral ores, recyclable wastes, contaminated water and soils, and toxic wastes, metal contaminants and the like may now be extracted therefrom using conventional processes in large recovery quantities heretofore un known in the art.
As will be appreciated by those skilled in the art, it has also been found that the startling quantities of metallic elements extracted and recovered from starting materials contemplated hereunder are attributable to the compositions of the present invention preventing re-bonding or reformation of complexed structures before the subsequent extraction process or processes may be performed. That is, the contacting step of starting materials with compositions of the present invention transcends conventional leaching; the present invention enables the underlying complexed structure of contaminant materials and the like to be essentially permanently decomposed into simpler components that may effectively extract and remove metal elements and the like with an efficiency heretofore unknown in the art. As is well known in the art, by xe2x80x9ccomplexingxe2x80x9d is meant the presence of elements as mineral elements or charged ions that are solvated, chelated, or otherwise implicated or encapsulated in a structural arrangement characterized by a matrix of covalent bonds or the like.
Practitioners in the art will, of course, recognize that prior limitations upon the extent of extraction and removal of metals and the like from starting materials contemplated hereunder have caused inaccurate assays, i.e., underestimated assays, of the presence of contaminants and impurities contained therein. The unique ability of the present invention to break down covalent bonds and the like populating underlying structures of starting materials and the correlated ability to sustain this breakdown for sufficient duration to enable subsequent conventional extraction and removal procedures to be performed has enabled higher percentages of the formerly underestimated metallic elements to be extracted and reported.
It will be understood that, absent contacting treatment using an attacking and aggressive liquor reagent as taught by the present invention, if the conditions under which conventional leaching operations are performed are not carefully controlledxe2x80x94particularly with regard to pH and temperaturexe2x80x94then re-complexing of the underlying structure of the starting material appears to occur due to reformation of covalent bonds or the like. But, applying a suitable composition of caustic silica solution as disclosed herein has been found to allow identification and assay of elements hereinbefore missed because of the limitations of the art. It is, of course, well known in the art that the concentration of compositions of the present invention, in conjunction with the contacting time, temperature, pressure, and pH determine the structural impact upon starting materials.
Thus, without restricting the present invention to any particular theory, it is postulated that a suitably formulated caustic silicate solution composition chemically alters the bonds within the molecules and/or compounds constituting the underlying structure of the starting material, thereby allowing certain elements, such as lead, gold, cadmium, and/or zinc, to be permanently disengaged therefrom and then readily solubilized and subsequently removed via a plurality of conventional extraction techniques. In addition, it is postulated that compositions of the present invention chemically alter the bonds within the molecules and/or compounds of the starting material, thereby allowing certain elements to be more accessible to and removed by conventional extraction processes. It is further postulated that certain atoms of particular elements are removed from the structure of the starting material, and that resulting vacancies within this structure are satisfied by replacement elements. For example, a hazardous or toxic element such as lead is removed from the molecular structure, and in other cases a valuable element such as a noble metal, e.g., gold, and is replaced by another element, preferably a non-hazardous, non-toxic, inexpensive element.
The compositions of the present invention are based upon the presence of a caustic silica solution. The term xe2x80x9ccaustic silica solutionxe2x80x9d as used herein relates to a caustic aqueous solution, i.e., an aqueous solution with a pH greater than about 8, more preferably with a pH greater than about 9, still more preferably with a pH greater than about 10, and still more preferably with a pH greater than about 11) in which silica (i.e., SiO2) has been dissolved.
Examples of caustic aqueous solutions include solutions of bases, including for example, solutions of strong Bronsted bases. Such bases include alkali metal hydroxides, particularly sodium hydroxide, i.e., NaOH, or potassium hydroxide, i.e., KOH, and ammonia, i.e., NH3. Preferably, the caustic aqueous solution contemplated by the present invention comprises sodium hydroxide.
Sodium hydroxide, i.e., NaOH; xcx9c40 grams/mole, is a water soluble hydroxide of an alkali metal, and is often referred to as a strong Bronsted base in aqueous solution. Another example of such an alkali hydroxide is potassium hydroxide, i.e., KOH. At 25xc2x0 C., the pH of an aqueous solution of NaOH may be approximated as pHxcx9c14+log10[NaOH], where [NaOH] denotes the concentration of NaOH in units of moles per liter. An aqueous solution of NaOH with a concentration of 1.0 molar, i.e., moles/liter, therefore has a pH of approximately 14; similarly, concentrations of 0.1, 0.01, 0.001, and 0.0001 molar NaOH yield approximate pH values of about 13, 12, 11, and 10, respectively. A 1.0 molar NaOH solution may be prepared by dissolving 1.0 mole of NaOH, i.e., xcx9c40 grams NaOH, in enough water to yield 1.0 liters of solution. In contrast, a 1.0 molal NaOH solution may be prepared by dissolving 1.0 mole of NaOH, i.e., xcx9c40 grams NaOH, in 1 kilogram of water. Concentrations are often described in units of percent weight by weight, i.e., % w/w; for the purposes of this disclosure, a concentration so denoted is analogous to molality. For example, a 10% w/w NaOH aqueous solution may be prepared by dissolving 100 grams of NaOH, i.e., xcx9c2.5 moles, in 1000 grams of water.
Alkali metal hydroxides, such as sodium hydroxide and potassium hydroxide, may be obtained from a wide variety of commercial suppliers in various forms, e.g., as a solid or an aqueous solution, and in various states of purity. Alkali metal hydroxides of moderate to high to very high purity may be used for the present invention, provided the contaminants do not significantly interfere. Preferably, the purity is greater than 90%, more preferably greater than 95%, still more preferably greater than 97%, yet still more preferably greater than 99%.
Preferred caustic silica solutions which have been found to be useful under the practice of the present invention comprise NaOH in a concentration from about 1% w/w to about 60% w/wxe2x80x94having a resulting pH of about 12-16. In some preferred embodiments, caustic silica solutions comprise NaOH in a concentration from about 1% w/w to about 30% w/wxe2x80x94having a resulting pH of about 12-14; more preferably, from about 5% w/w to about 25% w/wxe2x80x94having a resulting pH of about 13-14. The pH of the caustic silica solution may be measured using any known method, including, for example, optical, e.g., colored pH indicators or pH test paper, or electrical, e.g., electronic pH meters, methods. The pH of the caustic silica solution taught by the present invention should preferably be greater than about 10.3, more preferably greater than about 12, and still more preferably greater than about 13.
Embodiments of the caustic silica solutions useful in the practice of the present invention also comprise dissolved silica, i.e., SiO2. The physical and chemical properties of silica, also known as silicon dioxide, are variously described in the published literature; see, for example, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition (John Wiley and Sons, 1978), vol. 20, pp. 748-781; and The Chemistry of Silica, by Ralph K. Iler (John Wiley and Sons, 1979), especially Chapter 1. Silica may be found in crystalline or amorphous forms, and the term xe2x80x9csilicaxe2x80x9d is also often used to refer to silica which has been hydrated or hydroxylated to a greater or lesser degree; that is, to yield colloidal silica and silica gels. At ordinary temperatures, silica is chemically resistant to many common reagents. Common aqueous acids do not attack silica, except for hydrofluoric acid, i.e., HF, which forms fluorosilicate ions, i.e., SiF6xe2x88x922. Different forms of silica have varying solubilities in water. For example, the solubility of quartz, e.g., a crystalline silica, at room temperature is about 6 ppm reported as SiO2, i.e., 0.1 mmol/kg, whereas the solubility of many amorphous silicas is somewhat larger, from about 80 to about 130 ppm reported as SiO2, i.e., 1.4 to 2.2 mmol/kg. Silica solubility increases with temperature, approaching a maximum at about 200xc2x0 C.
An aqueous solution in equilibrium with amorphous silica at ordinary temperatures contains monomeric monosilicic acid (i.e., Si(OH)4) also known as orthosilicic acid or silica hydrate (and is sometimes referred to as H4SiO4 or H2SiO3):
SiO2(S)+H2O(1)xe2x86x92H2SiO3(aq)
SiO2(S)+2H2O(1)xe2x86x92H4SiO4(aq)
This acid is dibasic, dissociating in two steps:
The first conjugate base, SiO(OH)3xe2x88x921, is known as metasilicic acid, and is sometimes also referred to as H3SiO4xe2x88x921 or HSiO3xe2x88x921. The solubility of amorphous silica appears to be a minimum at about pH 7 and increases markedly above pH 9. Below pH 9, the solubility is independent of pH; above pH 9, the solubility increases because of increased ionization of silicic acid. Above pH 9, species such as H3SiO4xe2x88x921 and the dimers Si2O2(OH)5xe2x88x921 and Si2O3(OH)4xe2x88x922 are important. Above pH 11, species such as H2SiO4xe2x88x922 and the dimers Si2O4(OH)3xe2x88x923 are important. For example, the solubility of amorphous silica at 25xc2x0 C. is reported as 138, 180, 310, and 876 ppm (reported as SiO2) for pH values of 9, 9.5, 10, and 10.6, respectively.
Dissolved silica may be precipitated from a saturated or supersaturated solution to form amorphous silica, or it may undergo polymerization to give discrete particles which associate to give chains and networks, such as gels, or which grow in size and decrease in number to yield sols, such as colloidal silica. The rates of precipitation and polymerization are dependent on pH and salt concentration.
Preferred caustic silica solutions which are useful in the practice of the present invention comprise dissolved silica. Preferred caustic silica solutions comprise dissolved silica in a sufficient saturating concentration from about 0.001% to about 5% w,w (about 10 ppm to about 50,000 ppm), more preferably from about 0.001% to about 1% w/w (about 10 ppm to about 10,000 ppm), still more preferably from about 0.01% to about 0.5% wlw (about 100 ppm to about 5,000 ppm), and even more preferably about 0. 1% w/w (about 1000 ppm). In this context, units of ppm are related to units of % w/w by the equation:
(ppm)=(% w/w)xc3x97(10,000).
For example, a 1000 ppm SiO2 or 0.1% w/w SiO2 aqueous solution may be prepared by dissolving 1 gram of SiO2, i.e., 0.0167 moles, in 1000 grams of water.
Silica may be obtained from a wide variety of commercial suppliers, usually as the solid, in various forms and various states of purity. Preferably, the caustic silica solutions useful in the present invention are prepared using amorphous silica. Preferably, the purity of the silica is greater than 90%, more preferably greater than 95%, still more preferably greater than 97%, yet still more preferably greater than 99%.
In order to prevent or minimize contamination and/or interference by adventitious components, the caustic silica solutions embodying the present invention are preferably prepared using purified water, such as distilled water, more preferably deionized distilled water.
The caustic silica solutions embodying the present invention may be prepared by any of a variety of known methods. For example, one part by weight of high purity, dry sodium hydroxide may be mixed with one part by weight of high purity, dry silica and three parts by weight of distilled deionized water in a suitable container such as a glass beaker or stainless steel vessel. To expedite preparation of the caustic silica solution, the mixture may be heated, for example, to approximately 100xc2x0 C., using a hot plate or the like, and stirred occasionally or continuously using a mechanical paddle or magnetic stirring bar or the like. It may then be advantageous to add purified water to the mixture, as needed, to maintain the original volume. So treated, a suitable caustic silica solution may be obtained after a period of about 30 minutes to about 4 hours. If the solution were heated during preparation, it may be advantageous to allow it to cool to room temperature. It may be preferred to filter the solution using any suitable known means to remove some or all of any remaining undissolved material, for example, undissolved silica. For instance, the cool solution may be filtered through Whatman(copyright) Filter Paper#1 using a water aspirated Buchner funnel. The concentration of dissolved silica may be determined using atomic absorption or calorimetric analysis using a molybdic acid reagent or the like.
For embodiments of caustic silica solutions possessing relatively high concentrations of silicaxe2x80x94approaching 1%xe2x80x94the presence of dissolved silica may be verified by titration with hydrochloric acid or another suitable acid to a pH of about 4-6, causing the dissolved silica to polymerize. The solution becomes cloudy and a silica gel forms.
It has been found that caustic silica solutions of the present invention should preferably be stored in an airtight container to prevent degradation. For example, degradation may be caused by adventitious carbon dioxide which dissolves to form carbonic acid, H2CO3, and then may react with and thereby neutralize dissolved NaOH to form water and Na2CO3. Accordingly, caustic silica solutions embodying the present invention should preferably be stored in an inert, or relatively inert, container. As will be understood by those skilled in the art, for long term storage, materials such as glass should be avoided; instead plastic containers, such as polyethylene bottles or barrels, are preferred. So stored, it has been found that caustic silica solution embodiments contemplated hereunder are stable for a period of at least several months.
It should be clearly understood by those skilled in the art that use of an embodiment of the present invention introduce an aggressive, assaultative chemical processing step applied initially in the course of an extraction and removal procedure, preferably involving prolonged contacting between a starting material and the embodiment comprising a caustic silica solution composition.
The starting material may be contacted with this caustic silica solution liquor by simple mixing. If the starting material is a solid, or contains solid materials, a slurry of starting material and caustic silica solution may be prepared. Suitable proportions of starting material to caustic silica solution may easily be determined by the skilled artisan. For solid starting materials, such as soils, enough caustic silica solution may be added to form a slurry with acceptable handling properties, such as consistency and flow properties. For example, for a soil sample, proportions of approximately 5 liters of caustic silica solution per kilogram of soil have been found to be appropriate. For more dense materials, such as crushed ores, proportions of approximately 10 liters of caustic silica solution per kilogram of material have been found to be appropriate.
Once combined, the mixture may be allowed to stand undisturbed or it may be further mixed, stirred, or agitated. The mixture may be maintained at a temperature of from about 10 to about 200xc2x0 C. In some preferred embodiments, the mixture is maintained at room temperature; in others, the mixture is heated. In some preferred embodiments, the mixture is heated and pressurized, for example, at a pressure of about 1 to 50 atmospheres, i.e.,. about 0.1 to 5 MPa. Once this admixture has been sufficiently combined, chemical processing taught by the present invention may continue for a period of minutes to days, more preferably for a period from about 10 minutes to about 6 hours, still more preferably for a period of about 1.5 hours. It will be understood that it is imperative for sufficient time to be allocated this contacting procedure in order to effectuate the breakdown of the starting material""s underlying complexed structures so that subsequent extraction and removal of metal impurities and contaminants and the like may be accomplished at extreme levels hereinbefore unknown in the art.
After the appropriate period for this chemical processing under the influence of an embodiment of the caustic silica solution, it is contemplated that the treated admixture will be further processed by well known standard extraction means. For example, the solids of the mixture are separated from the liquid of the mixture. Typically, the mixture is filtered to remove the solids, i.e., treated material, solid residue, and yield a filtrate, i.e., extract, liquid extract. Any suitable filtration or separation method may be used. For example, the mixture may be filtered through a filter media, such as a filter paper or filter pad. Alternatively, the mixture may be separated by centrifugation or other mechanical means.
It should be clear that the key to the success of the present invention regarding extraction and removal of metals and other elements from contaminated materials and the like is that it triggers a powerful chemical process that alters the unerlying bonding structures of the starting material. It has been observed, in some instances, that elements in the starting material are solubilized. In other instances, it appears that elements remain in the residue, but are rendered more exposed and therefore more readily extracted using conventional processes. It has further been observed that the present invention prevents at least significant or noticeable re-formation of the bonding that constitutes the complexed structure of the starting material.
For example, it has been observed that chemical processing with a caustic silica solution taught by embodiments of the present invention as applied to a lead-containing material causes a large portion of the lead to be solubilized, i.e., placed in solution. Similarly, it has been observed that such chemical processing of a gold-containing material causes a large proportion of the gold to be solubilized. It will, of course, be understood that a solubilized element may be present in solution as a neutral element or as a charged ionic species, and may further be solvated, chelated or otherwise complexed by solvent, chelating agents, or complexing agents also present.
Once permanently solubilized as contemplated hereunder, the uncomplexed, susceptable elements can be removed from the extract solution using known methods. For example, dissolved elements, such as lead in the ionic form, Pb+2, may be removed by precipitation in the form of an insoluble or sparingly soluble salt, e.g., lead sulfate (PbSO4), by adding a suitable soluble salt, e.g., a sulfate such as sodium sulfate (Na2SO4). The solubility product of lead sulfate is approximately 1.06xc3x9710xe2x88x928 at 18xc2x0 C.; the solubility in water is therefore about 10xe2x88x924 moles per liter. Thus, by adding sodium sulfate until further precipitation is minimal, the residual lead ion concentration will be substantially reduced.
In an alternative subsequent extraction process, a solubilized species, such as the lead ion, i.e., Pb+2, may be removed from the extract solution using an electrowinning process, for example, by electrolyzing the solution, or by adding another material with a standard electrode potential more negative than the standard electrode potential of the metal to be precipitated. For example, the standard electrode potential for lead (i.e., Pb+2+2exe2x88x921xe2x86x92Pb) at 25xc2x0 C. (as measured versus a normal hydrogen electrode) is Exc2x0=xe2x88x920.1263 volts whereas the standard electrode potential for aluminum (i.e., Al+3 +3exe2x88x921xe2x86x92Al) at 25xc2x0 C. (as measured versus a normal hydrogen electrode, in 0.1 M NaOH) is Exc2x0=xe2x88x921.706 volts. See, for example, Millazzo and Caroli, Tables of Standard Electrode Potentials, Wiley-Interscience, New York, 1977. The electrode potential for aluminum is more negative than the electrode potential for lead (even at low lead ion concentrations). Thus aluminum metal will reduce lead ion according to the reaction 3Pb+2+2Alxe2x86x923 Pb+2Al+3, the lead ion will precipitate as lead metal (often in the form of a sponge-like material) and the aluminum metal will be solubilized as aluminum ions. For optimal electrowinning, the aluminum metal is preferably powdered or otherwise divided, so as to promote reaction. Other materials with suitable electrode potentials may also be used. For example, the standard electrode potential for manganese (i.e., Mn+2+2exe2x88x921xe2x86x92Mn) at 25xc2x0 C. (as measured versus a normal hydrogen electrode) is Exc2x0=xe2x88x921.029 volts, and may be predicted to be useful for electrowinning lead ions from solution.
Similarly, the standard electrode potential for the gold(I) ion, i.e., Au+1+exe2x88x921xe2x86x92Au, at 25xc2x0 C. (as measured versus a normal hydrogen electrode) is Exc2x0=+1.68 volts and the standard electrode potential for the gold(III) ion, i.e., Au+3 +2exe2x88x921 xe2x86x92Au+1, ) at 25xc2x0 C. (as measured versus a normal hydrogen electrode) is Exc2x0=+1.29 volts. The electrode potential for aluminum is more negative than the electrode potentials for gold (even at low gold ion concentrations). Thus aluminum metal will reduce gold ions, which will precipitate as gold metal as the aluminum metal is solubilized as aluminum ions.
Also, the standard electrode potential for the zinc, i.e., Zn+2 +2exe2x88x921 xe2x86x92 Zn, at 25xc2x0 C. (as measured versus a normal hydrogen electrode) is Exc2x0=xe2x88x920.763 volts, and may be predicted to be useful for electrowinning lead ions and/or gold ions from solution.
It will be appreciated that the caustic silica solution compositions of the present invention have been found to facilitate the efficient and greatly enhance extraction and/or recovery of elements from starting materials such as mineral ores, recyclable wastes, contaminated soils, toxic wastes, e.g., listed and/or characteristic toxic wastes, and other materials. An example of a toxic waste is dust produced through steelmaking processes such as electric arc furnace dusts and BOP dusts.
It will also be appreciated that the caustic silica solution compositions of the present invention have been found to facilitate the efficient and greatly enhance the extraction and/or recovery of elements, more preferably metals, still more preferably heavy metals (i.e., metals with specific gravity of about 5.0 g/cm3 or higher, such as chromium, vanadium, molybdenum, zirconium, zinc), noble metals (e.g., gold, silver, platinum), platinum group metals (e.g., platinum, palladium, nickel) group metals, and toxic metals (e.g., aluminum, cadmium, arsenic, lead, silver, mercury, barium, selenium), and other metals (e.g., titanium).